1. Field of the Invention
The present invention relates generally to semiconductor devices, and more particularly to wire-bond free, white light emitting devices and methods for fabricating the same.
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
Conventional LEDs cannot generate white light from their active layers. Light from a blue emitting LED has been converted to white light by surrounding the LED with a yellow phosphor, polymer or dye, with a typical phosphor being cerium-doped yttrium aluminum garnet (Ce:YAG). [See Nichia Corp. white LED, Part No. NSPW300BS, NSPW312BS, etc.; See also U.S. Pat. No. 5,959,316 to Lowrey, “Multiple Encapsulation of Phosphor-LED Devices”]. The surrounding phosphor material down-converts the wavelength of some of the LED's blue light, changing its color to yellow. Some of the blue light passes through the phosphor without being changed while a substantial portion of the light is down-converted to yellow. The LED emits both blue and yellow light, which combine to provide a white light. In another approach light from a violet or ultraviolet emitting LED has been converted to white light by surrounding the LED with multicolor phosphors or dyes.
One method used to fabricate efficient semiconductor devices is called flip-chip, or wire-bond free, mounting. Flip-chip mounting of LEDs involves mounting the LED onto a submount substrate-side up. Light is then extracted and emitted through the transparent substrate, or the substrate may be removed altogether. Flip-chip mounting is an especially desirable technique for mounting SiC-based LEDs. Since SiC has a higher index of refraction than GaN, light generated in the active region does not internally reflect (i.e. reflect back into the GaN-based layers) at the GaN/SiC interface. Flip-chip mounting of SiC-based LEDs offers improved light extraction when employing certain chip-shaping techniques known in the art. Flip-chip packaging of SiC LEDs has other benefits as well, such as improved heat extraction/dissipation, which may be desirable depending on the particular application for the chip.
Various approaches for the fabrication of white-emitting, flip-chip mounted devices have been attempted. For example, Philips Lumileds has developed a thin film flip-chip technology which they combine with their “Lumiramic” phosphor plates to create white-emitting, wire-bond free LEDs. To manufacture these devices, multiple fabrication steps are carried out at the singulated chip level. Base LEDs are individually flip-chip mounted to submount wafers prior to removing the LED substrate. The Lumiramic plates are then bonded to each chip individually.
As another example, in U.S. Patent Pub. No. 2008/0173884 (assigned to the same assignee as the present invention), the wire-bond free wafer is coated with phosphor-loaded silicone and then cured after it is fabricated. The coated wafers can then be further processed by grinding down the phosphor coating to a uniform thickness. Finally, the wafers are diced, and the singulated chips are placed for die attachment.
While flip-chip, or wire-bond free, LED chip designs are suitable for a variety of applications, white-emitting flip-chip devices can be difficult to fabricate. To maximize the overall light extraction while limiting the light emission from the sidewalls of the devices, the LED wafer substrate may be thinned or entirely removed. As such, the mechanical support for the remaining material is essentially provided by: the electrodes, and the package or submount the devices are flip-chip mounted onto. This can result in devices that are mechanically delicate and subject to low-yield during fabrication and die handling. Reliability problems after packaging can also be an issue.
Additionally, the base LED materials (such as SiC, GaN, and various contact metals) may have relatively low coefficients of thermal expansion (CTE), such as in the <20 ppm/° C. range. Alternatively, some of the materials provided to make the chips emit white light (such as silicone coating) have high CTE, such as in the >100 ppm/° C. range. The low CTE and high CTE materials are typically attached to one another to form a completed device, thus creating a CTE mismatch. The CTE mismatch may exacerbate the mechanically delicate nature of the white-emitting, flip-chip devices. This may lead to device failure during temperature cycles.